Wednesday, May 25, 2016

IGZO: More than Displays

Oxide semiconductors, alongside low-temperature polysilicon,
have been the high mobility alternative to amorphous silicon backplanes for a
few years now. Flexible display manufacturers in particular seem to have embraced
oxide semiconductors for future generations and new form factors. It’s
interesting to note that oxide semiconductors have other primary uses besides
displays, in large part due to their extremely low leakage current values.

I talked with Johan Bergquist (see photo below) of Semiconductor Energy
Laboratory (SEL) about these uses and how they relate to SEL’s C-axis aligned
crystalline indium gallium zinc oxide (CAAC IGZO) technology. Within SID, SEL
is best known for its many advances in active backplane technologies. Most
recently, SEL has demonstrated high-definition CAAC IGZO displays, both on
glass and flexible substrates. The president of SEL, Dr. Shunpei Yamazaki, is recognized
in many societies, including SID, for this work. Beyond displays, SEL has shown
the capability of CAAC IGZO in representative circuits, such as system-on-glass
processing units and multi-level memory circuits. In fact, large-scale
integration (LSI) applications are some of the most promising uses for
IGZO.

“IGZO doesn’t have the short gate effect that silicon has,”
said Bergquist. “This means that reducing design rules does not reduce the
mobility, and SEL has in fact demonstrated 30 nm technology in CAAC IGZO.” CAAC
IGZO is aligned in the transverse direction, while it is nanocrystalline in the
lateral dimensions. Because the mobility is still below that of crystalline
silicon, oxide semiconductors are, unlike CPUs and other circuits requiring
fast switching, best suited for circuits that can utilize the
next-to-nonexistent leakage current, such as nonvolatile memories. These can be
used, for instance, as register backup memories and frame buffers. Because the
technology is analog in nature, multilevel memories are possible. “SEL’s goal
is to make an 8-bit memory, but we are not there quite yet. 4-bit and 5-bit
memories have been realized, though,” says Bergquist. “The trick is to use
hybrid circuits where silicon and IGZO are used together.”

At the moment, the reliability of oxide semiconductors is
not at the same level as that of silicon-based circuits, so memory chips based
on oxide logic are not available. There is a lot of promise in IGZO circuits,
especially integrating these with oxide semiconductor backplane displays, and
future efforts with chip maker partners of SEL will likely see to it that
product reliability will reach silicon levels. We may have to wait for a few
years before this happens but until then, we can enjoy the benefits of IGZO in
fantastic mobile-sized displays, and beyond these, moderate-sized high-pixel-density
displays, both on glass and flexible substrates. -- Jyrki Kimmel for
Information Display